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Dark matter may have been found on Earth

By Anil Ananthaswamy

Particles of invisible “dark matter” have been detected deep inside a mountain in Italy, a collaboration of Italian and Chinese physicists claims. But others remain sceptical of the result, because other experiments have failed to detect any dark matter at all.

On Wednesday 16 April, at a workshop in Venice, Italy, the Dark Matter (DAMA) collaboration announced the results of the 4-year second phase of its experiment. DAMA scientists claimed to see dark matter back in 2003, but some scientists believed the result was a quirk of statistics. Now the evidence is stronger.

“We are pretty sure now that this [signal] is not a statistical fluke. What it means is another matter,” says Francis Halzen, an astroparticle physicist at the University of Wisconsin-Madison, US. He spoke to New Scientist after attending the announcement by DAMA project leader Rita Bernabei of the University of Rome, Italy.

Unidentified substance

Astronomers believe our galaxy is awash with particles of dark matter, the invisible, unidentified substance that makes up nearly 90% of the matter in the universe. So far, the existence of dark matter in space has only been determined by its gravitational pull on normal stars and galaxies.

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The DAMA experiment has looked more directly for dark matter particles hitting the Earth. The experiment takes place in an underground laboratory that lies beneath 1.4 kilometres of rock, inside the Gran Sasso mountain in Italy. The team looks for flashes of light in a sodium iodide detector.

The flashes mainly come from background “noise”, such as ordinary neutrons from radioactivity in the surrounding rock. But some might also come from dark matter particles, and, if so, the scientists expect to see seasonal variations in the signal because the Earth’s speed through our galaxy changes depending on its direction of motion.

This theory predicts that the Earth should be hit by more dark matter particles in June, when it is moving through the galaxy in the same direction as the Sun. There would also be fewer particles in December, when it is moving in the opposite direction.

Intense scepticism

That’s exactly what the DAMA team reported in 2003, following the first phase of their experiment, which ran for 7 years with a 100-kilogram detector. But the results were met with intense scepticism, as none of the other experiments looking for dark matter had seen anything.

So the DAMA team renewed their search with a larger 250-kilogram detector. And they say they can now confirm that the new experiment has again shown an annual variation in the number of particles hitting their detector. There is a slight increase above the average in June rate and a corresponding decrease in December.

The team claims that the new result is highly significant. The odds that they are simply seeing a random fluctuation are less than one in several billion, they say. They also say they have ruled out the possibility that the signal is due to some systematic effect, such as seasonal variations in the temperature of their underground cavern.

But Halzen is wary. “The discussion about whether this is some unknown systematic effect remains,” he says.

Richard Gaitskell from Brown University at Providence, Rhode Island, US, and a member of two dark matter experiments – the Cryogenic Dark Matter Search (CDMS) and the Xenon project – also remains sceptical, because no other experiment has seen signs of dark matter.

“Right now, it is very difficult to reconcile theoretically what they are seeing and what we are seeing,” says Gaitskell.

But both Halzen and Gaitskell agree that the new DAMA results might prompt others to try and duplicate the results. “The issue of dark matter is important enough that we should pay attention to this; we should not just ignore it,” says Halzen.